SHOOT-ROOT ALLOCATION WITH RESPECT TO C, N AND P - AN INVESTIGATION AND COMPARISON OF RESISTANCE AND TELEONOMIC MODELS

Two shoot:root allocation models are described: the transport-resistance approach, and a teleonomic (goal-seeking) method based on maximizing specific growth rate when the system is growing exponentially. These are applied to two growth modes: exponential growth, and the steady state where all variables are constant with no net growth. The dynamic behaviour after shoot defoliation is investigated: the damping/overshoot effects observed are highly dependent on the presence or absence of product inhibition of the input process (e.g. plant substrate N may inhibit the uptake of mineral N by the plant). The teleonomic model is far more damped than the resistance model and may therefore be misleading if used to interpret transient experiments. Ontogenetic effects on allocation are simulated by varying the scaling (with plant size) of the transport resistances; this may give increasing allocation to the shoot or the root with the passage of time. The two models resemble each other very closely as far as equilibrium responses are concerned - this applies to exponential growth and to the steady state. Increasing the nitrogen input may lead to lower or higher whole-plant carbohydrate levels. The response to increasing nitrogen input depends on the other inputs; for instance it can be much curtailed by low phosphorus inputs. The response to phosphorus input can be similarly limited.